I've been playing with some filaments recently that are extremely brittle and take very little to snap (mostly translucent, colored ones). A minor kink in the filament, combined with high speed / high frequency infill is enough to make it snap mid-print, usually on longer prints. Sometimes within the extruder, sometimes above it (usually within).

I've tried printing out the large herringbone gear for Wade's three times now, all three a failure, due to filament snapping in the middle of the print job. It's starting to get more than a little annoying at this point...

Does anyone have any ideas on how to prevent it, apart from the obvious (switching to a different filament)?

Thing is, those very same, brittle filaments also tend to cool very quickly for some reason, which makes them perfect for overhangs, things like gear teeth etc. Also, the chances of snapping are directly proportional to the length of a print job, so i wouldn't say it's something cause by, say, pressure / extruder rate requirements or anything similar. And yes, i've tried loosening the idler on the extruder as much as i'm comfortable with. I don't think loosening it any more would help much.

Edit: Tried hanging the filament loop on a desk lamp and giving it the gentlest possible approach too. Didn't help. I think the problem is compounded by the fact the loop of filament i had seems to have been taken from the end of a reel, so the radius of the loops is pretty small and curvature is huge.

I've left the bot off with the filament still in.I go out for half an hour, get back and the filament i left looped around the lamp is gone.I take a better look and notice there's about 10cm of filament sticking out the extruder, look around and find the loops on the floor.

Apparently, it just snapped off its own accord, while at rest, and catapulted the loop off the lamp.

I would try a few things:Sheath the filament in a PTFE or similar tube and attach one end to the X carriage so that it doesn't move, and the end of the tube is aligned with the entry point on your extruder. The idea being to relieve any stress on the cable and increase the bend radius to eliminate any stress points.The other thing you could do is warm the filament very slightly (an oven?) and then re-wind onto something with a bigger diameter.If you feel like the feed mechanism of your extruder is heating up you might want to point a small fan at it.

Thanks for the ideas.The idea behind winding it around the desk lamp was actually half-inspired by the fact the lamp heats up. Not by a lot (CCFL), but it is pretty warm. I thought that might relieve some stress by letting it stretch more as it unwinds from the loop, but no luck.

The PTFE tube idea sounds good, i'll try to find something suitable.I'm nearly done with this loop of PLA, going to see if the translucent sky blue snaps as easily. At least that one was wound in a much bigger radius, so it has that going for it.

I've just examined the translucent red filament under bright light...It's full of transversal striations in places. It seems like it was pre-strained and really does take very very little to snap.

As i was nearing the end of the loop (i keep writing "loop" because it's not a reel, it was just some 5m or so worth of sample, courtesy of 2printbeta), it started snapping more and more often, culminating in snaps every 10cm or so. I guess it's not normally like that, since i printed some stuff just fine with the first few meters, but the end got mangled up when it was being extruded onto the reel.

Switched to translucent blue now, so far so good.

It's interesting how different the red is. It's a deep ruby color, not washed out like most PLA colors. And it's completely transparent.So transparent that the legs of the lion statue i was printing were looking like a solid part, not a bunch of layers - light was getting cleanly refracted through it. More like polycarbonate, i guess.